This application is related to Japanese Patent Applications No. Hei 11(1999)-268055 filed on Sep. 22, 1999 and Hei 11(1999)-268056 filed on Sep. 22, 1999, whose priorities are claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic type copying machine, laser beam printer, facsimile machine or the like which uses two-component developing agent comprising toner and carriers, and particularly to an image forming apparatus which can correct the specific concentration of toner in accordance with variation of humidity to stably form an image.
2. Description of the Related Art
A conventional image forming apparatus is equipped with a photosensitive drum for carrying an electrostatic latent image thereon, which is disposed in the housing of the apparatus. The surface of the photosensitive drum is uniformly electrified by an electrifying unit, and then an electrostatic latent image is formed on the surface of the photosensitive drum thus electrified. The electrostatic latent image formed on the surface of the photosensitive drum is developed with toner by a developing unit to form a toner image. The toner image is transferred onto a sheet serving as a transfer medium fed to a transfer unit.
Subsequently, the sheet is peeled off from the photosensitive drum by a peeling unit and then the toner image is fixed onto the sheet by a fixing unit. In such a conventional image forming apparatus, two-component developing agent comprising toner and carriers is used as developing agent of the developing unit. The toner is formed by dispersing colored pigment into resin, and the carriers are composed of ferromagnetic particles, for example, iron powder or the like. In the two-component developing agent comprising the toner and the carriers thus formed, the weight ratio (weight percentage) of the toner is equal to several %, and the toner of the developing agent is wasted every time an electrostatic latent image on the photosensitive drum is developed. Accordingly, the weight ratio of the toner in the developing agent, that is, the specific concentration of the toner must be kept in a predetermined range.
FIG. 7 is a block diagram showing a circuit for controlling the specific concentration of toner in the conventional image forming apparatus, and FIGS. 8 and 9 are graphs showing the output characteristics of an auto toner control sensor (hereinafter referred to as xe2x80x9cATC sensorxe2x80x9d) for the toner specific concentration.
As shown in FIG. 7, the toner specific concentration of the developing agent in a developing unit 101 is detected by a magnetic type ATC sensor 102, and CPU 103 controls toner supply from a toner cartridge 104 in accordance with detection information of the ATC sensor 102.
FIG. 8 shows the relationship between the toner specific concentration and the output value of the ATC sensor 102. When reduction of the toner specific concentration of the developing agent in the developing unit 101 is detected on the basis of the output value of the ATC sensor 102 by the CPU 103, CPU 103 drives a toner cartridge driving motor 106 to supplement toner into the developing unit through the toner cartridge 104. As a result of the toner supplement, the toner specific concentration increases, and the output voltage from the ATC sensor 102 is reduced. For example when the output voltage of the ATC sensor 102 is reduced to be less than a reference value V0 shown in FIG. 8 (for example, 2V), CPU 103 stops the driving motor 106 to stop the toner supplement from the toner cartridge 104, whereby the toner specific concentration of the developing agent in the developing unit 101 is kept to 4 wt % (reference value V0), for example.
The two-component developing agent in the developing unit 101 is liable to be influenced by humidity. Therefore, if no correction is carried out on the toner specific concentration in accordance with variation of humidity, the toner specific concentration increases as the atmosphere is more humid. On the other hand, the toner specific concentration is reduced as the atmosphere is less humid. This phenomenon occurs because the ATC sensor 102 detects the magnetic flux density to control the toner specific concentration. For example, when the atmosphere is more humid, the electrification amount of the developing agent is lowered, and the developing agent is kept more tight as a whole. Therefore, the ATC sensor 102 judges that the density of the carriers is high, that is, the toner specific concentration is low, and thus instructs to start the toner supplement.
On the other hand, when the atmosphere is less humid, the electrification amount of the developing agent is increased, and the developing agent is kept swollen. Therefore, the ATC sensor 102 judges that the density of the carriers is low, that is, the toner specific concentration is high, and thus instructs to stop the toner supply.
That is, the characteristic curve indicating the output of the ATC sensor of FIG. 8 is varied like a characteristic curve with respect to the variation of humidity as shown in FIG. 9. A stable image can not be obtained without correcting the above phenomenon
Consequently, when the atmosphere is kept in a high humidity state, the toner specific concentration of the developing agent rises up or the electrification amount is lowered. Therefore, toner scattering or background fogging (which means increase of background density and thus the background looks dirty) or defacement of images occurs, so that the image quality is lowered. On the other hand, when the atmosphere is kept in a low humidity state, the toner specific concentration of the developing agent is reduced or the electrification amount is increased. Accordingly, the image density is lowered, and for example, blurring of characters occurs, so that the image quality is lowered.
Furthermore, the developing agent is deteriorated with time lapse due to increase of the copy amount (copy frequency) and also increase of the developing agent stirring time, and due to the deterioration of the developing agent, the toner specific concentration rises up or the electrification amount is lowered as compared with the toner specific concentration at the initial stage of the developing agent, that is, when the device is set up or the developing agent has been just exchanged. Therefore, toner scattering or background fogging, or image defacement occurs, and thus the image quality is lowered.
In order to solve the problem, according to an image forming apparatus disclosed in Japanese Unexamined Patent Publication No. Hei 4(1992)-12380, a humidity sensor 105 is provided as shown in FIG. 7. If the humidity (relative humidity) is 70% or more, the toner supplement amount is controlled by a controller 103a so that the toner specific concentration of the developing agent in the developing unit 1 is equal to a first reference value (for example, 3.5 wt %). On the other hand, if the humidity is below 40% or less, the toner supplement amount is controlled by the controller 103a so that the toner specific concentration of the developing agent in the developing unit 1 is equal to a second reference value (for example, 4.5 wt %) higher than the first reference value.
FIG. 10 shows the operating characteristic of the humidity sensor 105 in the image forming apparatus.
In the image forming apparatus as described above, a control reference value (target value) indicated by a line C of FIG. 10 is provided. That is, under the humidity of 40% or less, the toner specific concentration is controlled to be equal to 4.5 wt %, and under the humidity of 70% or more, the toner specific concentration is controlled to be equal to 3.5 wt %.
However, actually, the toner specific concentration is shifted as indicated by A-line or B-line of FIG. 10 with variation of the humidity. That is, in FIG. 10, the A-line and B-line indicate the variations of the toner specific concentration with humidity variation when the humidity is equal to 55% at the setup time and when the humidity is equal to 69% at the setup time, respectively. Accordingly, it is difficult to control the toner specific concentration to the humidity so that the toner specific concentration is coincident with the control reference value indicated by the C-line. Therefore, the image forming apparatus thus constructed have the following problem.
That is, when the humidity range in which the toner specific concentration is corrected is set to 40% or less and 70% or more in advance, the transition of the toner specific concentration when the humidity at the developing agent initial time is equal to 55% is expected as indicated by the A-line of FIG. 10, and the transition of the toner specific concentration when the humidity at the developing agent initial time is equal to 69% is expected as indicated by the B-line of FIG. 10.
Accordingly, in the case where the toner specific concentration value is stored through the ATC sensor 102 into CPU at the developing agent initial time, the toner specific concentration under the humidity of 55% is equal to the reference value of 4 wt % (A-line) when the toner specific concentration value is stored on the assumption that the humidity at the developing agent initial time is equal to 55% (hereinafter referred to as xe2x80x9c55%-casexe2x80x9d), and also equal to the reference value of 4 wt % under the humidity of 69% (B-line) when the toner specific concentration value is stored on the assumption that the humidity at the developing agent initial time is equal to 69% (hereinafter referred to as xe2x80x9c69%-casexe2x80x9d).
When the humidity in each of the 55%-case and the 69%-case reaches 70%, the toner specific concentration in the 55%-case is equal to 3.5 wt % (A-line). However, even when humidity varies slightly, the toner specific concentration in the 69%-case must be corrected by the same amount, so that the toner specific concentration is set to 3 wt % (B-line).
As described above, when the image forming apparatus is set up or the developing agent is exchanged in the vicinity of the boundary between an area where the correction of the toner specific concentration should be carried out and an area where no correction of the toner specific concentration should be carried out, the toner specific concentration is greatly deviated from the reference toner specific concentration.
Furthermore, in the case where the toner specific concentration is corrected in accordance with the humidity variation, if the humidity is in the vicinity of the boundary between the area where the correction of the toner specific concentration is carried out and the area where the correction of the toner specific concentration is not carried out, the correction may be unintentionally carried out or not carried out due to a slight humidity variation. Therefore, the toner specific concentration is very unstable and thus it is difficult to achieve stable image quality.
For example, in the case where the image forming apparatus is set so that the toner specific concentration is corrected when the humidity is 70% or more, when the humidity varies like it is equal to 69% at a time, the next moment it varies to 70% and the next moment it varies to 69%, the toner specific concentration is carried out at some instantaneous time, but it is not carried out at the next instantaneous time, so that the toner specific concentration is very unstable.
Referring to FIG. 9, comparing both cases of 69% and 71% in humidity, the difference in toner specific concentration is equal to about 1 wt % between both cases. Therefore, if the correction of the toner specific concentration is carried out or not carried out every time the humidity varies by 1%, 2%, the toner specific concentration is very unstable.
In order to solve the above problem causing degradation of the image quality, Japanese Unexamined Patent Publication No. Sho 60(1985)-84557 discloses the following technique. According to this technique, the humidity sensor 105 is provided as shown in FIG. 7, and the variation of the toner specific concentration caused by the humidity variation is corrected by varying the reference value of the ATC output voltage in accordance with the variation of the humidity. In addition, the copy amount is counted, and when the copy amount thus counted reaches such a value that the developing agent is deteriorated and the output voltage of the ATC sensor is varied, the increment of the output voltage of the ATC sensor is added to the reference value of the output voltage of the ATC sensor to keep the toner specific concentration in the developing agent constant at all times. The copy amount is proportional to the stirring time of the developing agent, and the developing agent is deteriorated as the stirring time increases.
In the above publication, the reference value of the output voltage of the ATC sensor is shifted up in accordance with the copy amount to correct the toner specific concentration so that the toner specific concentration is kept constant, and also the correction is made so that the reference value of the output voltage of the ATC sensor is varied in accordance with the humidity variation. However, no consideration is paid to the time variation due to the copy amount, and the correction amount is determined on the basis of only the humidity variation.
That is, in the correction of the toner specific concentration as described above, even when the toner specific concentration is controlled to be constant, the correction amount for the humidity variation is not associated with the copy amount of the developing agent, but is kept constant as shown in FIG. 11 as the developing agent is deteriorated with increase of the stirring time of the developing agent (indicated by a broken line of FIG. 11), so that it is difficult to control the toner specific concentration constant for both of developing agent just after exchanged and developing agent which is being deteriorated.
FIG. 12 is a graph showing an example of the correction of the toner specific concentration in the conventional technique, and also shows the relationship between the toner specific concentration (wt %) and the ATC output value under three humid states (high humidity, normal humidity, low humidity). In this case, the toner specific concentration is corrected so that the ATC output reference V0 for controlling the toner specific concentration is shifted to Vb when the humidity is shifted to the high humidity side and to Vc when the humidity is shifted to the low humidity side.
However, since the correction amount is set to a fixed value over a use term of the developing agent (straight lines V0, Vb, Vc are in parallel to one another), the variation amount of the ATC output value at the low humidity side and the high humidity side is increased with increase of the stirring time of the developing agent as shown in FIG. 13 when the correction amount of the ATC output reference value at the high humidity side is set to +0.5V and the correction amount of the ATC output reference value at the low humidity side is set to xe2x88x920.5V.
Accordingly, the correction amount of the ATC output reference value lacks. For example, in the case of the developing agent at the developing agent stirring time of 120 h in FIG. 13, the correction amount of the ATC output reference value runs short by +0.4V (variation value of +0.9V) at the high humidity side and by xe2x88x920.4V (variation value of xe2x88x920.9V) at the low humidity side. Consequently, at the high humidity side, the ATC sensor output value is shifted from a point Vb (toner specific concentration: 4 wt %) to a point Vbxe2x80x2 (toner specific concentration: 4.4 wt %) and the toner specific concentration rises up by about 0.4 wt %. On the other hand, at the low humidity side, the ATC sensor output value is shifted from a point Vc (toner specific concentration: 4 wt %) to a point Vcxe2x80x2 (toner specific concentration: 3.6 wt %), and the toner specific concentration is reduced by about 0.4 wt %.
That is, in the case where the correction amount based on the humidity is proper at the time when developing agent just after exchange is used, the correction amount runs short for developing agent which has been used at a high copy frequency (copy number). At this time, when the humidity state is shifted to the high humidity side, the toner specific concentration rises up, and when the humidity state is shifted to the lower humidity side, the toner specific concentration is reduced.
On the other hand, in the case where the correction amount based on the humidity is proper when developing agent progressing in deterioration is used, the correction amount is excessive for developing agent just after exchange. At this time, when the humidity state is shifted to the high humidity side, the toner specific concentration is reduced, and when the humidity state is shifted to the low humidity side, the toner specific concentration rises up. Accordingly, it is difficult to keep the toner specific concentration constant with respect to the humidity variation over the using term of the developing agent, and thus the toner specific concentration is unstable with the humidity variation.
Further, in a multicolor image forming apparatus having plural image forming units using plural kinds of developing agents, color balance is important particularly when a full-color image or the like is desired, and thus it is necessary to properly correct the toner specific concentration for each of the plural kinds of developing agents. Therefore, the above problem is more critical when the image density correction or the toner specific concentration correction is carried out at plural times for every color.
In view of the above problems, the present invention has been made to provide an image forming apparatus characterized in that the toner specific concentration can be kept stable at all times during the period of using the developing agent irrespective of the humidity variation, so that an excellent image quality can be stably attained.
According to the present invention, there is provided an image forming apparatus comprising: a developing unit for stocking a developing agent containing toner and forming a toner image on a photosensitive medium with the developing agent; a toner supply unit for supplying the toner to the developing unit; a toner specific concentration detecting unit for detecting the toner specific concentration of the developing agent stocked in the developing unit; a humidity detecting means for detecting humidity in the neighborhood of the developing unit; a storage means for storing the humidity information detected by the humidity detecting unit; and a toner specific concentration correcting unit for controlling the toner supply amount of the toner supply unit on the basis of the humidity information stored to correct the toner specific concentration, wherein the toner specific concentration correcting unit compares newly-detected humidity with reference humidity corresponding to the humidity which is detected at the time when the developing agent is stocked into the developing unit or exchanged by new developing agent and which is stored in the storage unit, and corrects the toner specific concentration in accordance with the comparison result.
That is, the humidity (relative humidity) at the time when the developing agent is stocked or exchanged by new developing agent is set as the reference humidity, and the correction amount of the toner specific concentration is set in accordance with the variation amount from the reference humidity, so that the toner specific concentration can be surely corrected in accordance with the humidity variation.
In the present invention, the developing agent stocking time or the developing agent exchange time means the time point when the image forming apparatus is newly set up and the developing agent is prepared or the time point when deteriorated developing agent is exchanged by new developing agent, respectively. Specifically, it means the time just after the toner seal is broken and toner is poured into a developing agent tank or the time period during which the developing agent containing the toner poured in the developing agent tank is stirred.
In the present invention, the developing unit comprises a developing portion for forming a toner image on the photosensitive medium with the developing agent, a toner cartridge which is hermetically sealed so as to be openable while the developing agent containing at least the toner is stocked therein, and a developing agent feeding portion (for example, screw conveyor) for feeding the toner in the opened toner cartridge to the developing portion. The toner specific concentration unit corrects the toner specific concentration on the basis of reference humidity corresponding to the humidity of the interior and/or the exterior of the image forming apparatus which is detected by the humidity detecting unit just after the toner cartridge is opened.
The image forming apparatus of the present invention is further equipped with a stirring unit for stirring the developing agent stocked in the developing unit. The storage unit stores a stirring time of the stirring unit and a toner specific concentration which is newly detected by the toner specific concentration detecting unit, and the toner specific concentration correcting unit corrects the toner specific concentration on the basis of the stirring time and the comparison result for the humidity, whereby the toner specific concentration correction amount for the humidity variation is determined in accordance with the developing agent stirring time from the initial time (for example, the apparatus setup time or the developing agent exchange time). Accordingly, irrespective of the stirring time of the developing agent, the toner specific concentration can be accurately corrected in accordance with the humidity variation at all times.
In the image forming apparatus of the present invention, the storage unit stores the humidity at the time when the toner specific concentration is corrected as new reference humidity, and the toner specific concentration correcting unit compares the new reference humidity with humidity which is newly detected after the toner specific concentration is corrected and corrects the toner specific concentration in accordance with the comparison result, whereby the toner specific concentration correction value can be determined in accordance with the humidity variation from the humidity information at the initial time as reference humidity. Therefore, irrespective of the humidity atmosphere at the initial time (for example, the apparatus setup time or the developing agent exchange time), the toner specific concentration can be accurately corrected.
Further, the image forming apparatus of the present invention further comprises a voltage applying unit for applying voltage to the developing agent stocked in the developing unit to adjust the density of the toner image, an image density detecting unit for detecting the density of the toner image, and an image density control means for comparing the image density detected by the image density detecting unit with a predetermined reference image density and setting the voltage applied by the voltage applying unit in accordance with the comparison result, wherein the storage unit stores the applied voltage thus set and the detected humidity as new reference humidity, and the toner specific concentration correcting unit compares the applied voltage newly detected by the image density control unit with the previous applied voltage to judge the voltage correction direction, compares the newly detected humidity with the previous humidity to judge the humidity variation direction, and corrects the toner specific concentration only when the voltage correction direction corresponds to the humidity variation direction, whereby the toner specific concentration can be corrected only when the correction is actually needed.
In the image forming apparatus, the storage unit stores the applied voltage at the toner specific concentration correction time as a reference applied voltage, and the toner specific concentration correcting unit compares the applied voltage value newly set by the image density control unit with the reference applied voltage, corrects the toner specific concentration in accordance with the comparison result, and renews the reference applied voltage on the basis of the applied voltage at the toner specific concentration time, whereby the toner specific concentration correction can be executed only when the correction is actually needed.
According to the image forming apparatus of the present invention, the toner specific concentration correcting unit corrects the toner specific concentration only when the comparison result for the humidity is larger than a predetermined difference, and thus there does not occur such a situation that the correction of the toner specific concentration is carried out or not carried out due to a slight humidity variation, and thus the toner specific concentration can be corrected stably.
In the prior art, the correction of the toner specific concentration is carried out on the basis of the copy amount (copy frequency). However, by correcting the toner specific concentration on the basis of the stirring time of the developing agent, the variation of the toner specific concentration can be effectively used while the image density correction result and the toner specific concentration correction result are associated with each other. Therefore, the developing agent stirring operation can be prevented from being unnecessarily executed because of unnecessary execution of the toner specific density correction or image density correction, and reduction of availability and waste of expendables can be prevented.